Concept: Phyto-remediation

This sheet is part of the BOSS application.


Phyto-extraction (soil contamination)

In the direct in-situ removal of pollutants, (hyper) accumulating crops are cultivated. The crops are cut and transported for composting or controlled incineration. In farming areas, the intention is for these activities to be carried out by the farmers themselves.

The plant-types that are suited to this technique are those that have a high in-take capacity. In addition to this, it is important that the crops have a high return of (mowable) dry matter. If the crop has another economic and/or agricultural value, this is definitely a plus point.

Grasses and clover appear to be ideal for phyto-remediation because they have a fibrous root system which forms a permanently-compact rhizosphere (the soil in the immediate surrounding of the plant roots). This also provides the additional benefit that soil is protected against water and wind erosion. Literature makes reference to the following grasses: Alfalfa, clover, fescue grass, Bermuda grass and rye-grass.

Plants can also be transformed to selectively extract and accumulate heavy metals, so that potential metal-rich residues can be recycled. Transgenic tobacco and potato are good examples. By way of this transformation, the tolerance to heavy metals is also increased.

The possibility of increasing bio-degradation in plants is based on the fact that plants discharge certain organic substances (exudates) into the soil which stimulate bio-degradation in the following ways:

  • Stimulation of soil organisms in the rhizosphere.          
  • They contain enzymes that are able to transform organic compounds;
  • They are able to stimulate co-metabolic transformation of pollutants via micro-organisms.

As a follow-up to the clean up, regular ground sampling and analysis must be undertaken.

Hyper-accumulating crops are cultivated in the same way as other agricultural crops; thus they require the same labour-intensity and maintenance needs (regular follow-up of seeding, plants and harvests) Plant growth also requires sufficient follow-up.

No extra follow-up and maintenance is expected in the stimulation of bio-degradation.

Phyto-extraction (ground water pollution)

In-situ phyto-remediation of contaminated ground-water can take place via trees, which extract large volumes of ground-water. An example of this is poplars, which pump up an average of 260 litres per tree per day, which adds up to 4.2 million litres per hectare in 6 months. Once the ground-water has been extracted by the trees, further treatment takes place – such as degradation, volatilization or transformation.


Thus plants are used, possibility in combination with suitable soil additives, to establish the contaminants in the soil. This technique does not lead to the removal of the contaminants. The bio-availability of pollutants is thus lowered, with the main purpose being to prevent the further dispersion of pollutants.  A well enclosed vegetation surface hinders further spread via wind and water erosion, and leads to a drastic decrease in the percolation of the contaminant into the ground-water.

Plants that can be used for phyto-stabilisation must be able to tolerate the contaminants present in the soil as well as the specific growth conditions of the concerned site. They do not accumulate the contaminants, or only do so to a limited extent, in the upper parts in order to avoid spread to the food-chain.  They possess an extensive root system for good stabilisation of soil and for in-take of ground-water. Soil additives are often implemented for the immobilisation of heavy metals. These additives ensure the transformation of free and easily-soluble and exchangeable forms of metal into a more stable form. They also reduce the bio-availability for plants and the percolation into ground water.


The micro-organisms are situated mostly near the roots of the plants. Organic substances, such as mineral oil of PAC, can be removed by these micro-organisms. This technique can be used for the remediation of bio-piles which are seeded with plants, e.g. alfalfa (Luzerne).

Metabolic processes (in or ex-planta) can be driven by the plants itself. Contaminants can be adsorbed by the plant and degraded by enzymes from the plant, e.g. TNT-waste, BTEX and pesticides.


Implementation area and implementation conditions

Pollutant type                                                                                                                       

Cultivation of (hyper) accumulating crops is only suitable for pollutants that are taken-in by plants, such as heavy metals (cadmium, lead, copper, mercury, selenium, nickel, cobalt).

For in-situ phyto-remediation to take place, the pollutant must be found in the top-layer of the soil (the root zone). The concentration may not exceed the phyto-toxication level.

The technique will mainly be used in-situ for light large-scale diffuse pollutants like:

  • Deposits from industrial substances (e.g. metal industry and waste incineration installations).
  • Flood streams
  • Vehicle traffic
  • High-voltage cables and over-head wiring;
  • Surface elevation
  • Placing of GFT compost, cleansing or clearing sludge.

Ex-situ stimulation of bio-remediation is only applicable to biologically degradable pollutants. For volatile compounds there is yet another benefit; being that ‘tilling’ of the ground is not necessary for aeration because extra oxygen is brought into the soil by plants. Thus, almost all pollution is removed via bio-degradation and not by a combination of volatilisation and bio-degradation.

Soil type

Depending on the plant type, the type of soil will play a role. Plant growth can also be stimulated via fertilisation.

For large-scale diffuse heavy metal pollution, the cultivation of hyper-accumulating crops is the only removal technique that is practically usable. In such cases, a control varient is often implemented (e.g. liming). It is expected that sufficient results will be attained (to around the background value). However, this approach lasts for multiple seasons, possible decades (further research required).

The attained result from phyto-remediation relating to the stimulation of biological degradation is determined by the factors that influence biological degradation (pollutant type, soil type…). Phyto-remediation will only increase the speed of bio-degradation.



The method is regarded to be cost-effective as the pollution is removed b using in-situ biological treatment without high energy use, costly installations, production of wastes and/or consumption of secondary resources.

The exact costs depend on the type of execution; ex-situ treatment has higher costs because of excavation costs, costs for making the bio-piles,…

Mostly the costs lay between 20-100 € per m³. 


Environmental burden and measures to be implemented

No negative effects are expected from the cultivation of hyper-accumulating crops. Processing (composting or incinerating) takes place elsewhere; ash will probably need to be disposed.

The stimulation of biological degradation is not associated with any additional negative elements.

No negative aspects are expected on the safety front.